Here's the thing - this is not a problem with breastmilk. Formula-fed infants aren't getting enough Vit D, children drinking three cups of milk a day aren't getting enough Vit D, and adults arne't getting enough vitamin D.
That's because we're not designed to get adequate Vit D through food - so breastmilk isn't 'deficient' in it because it's not designed to transmit it in large quantities (it a bit like saying 'breastmilk is deficient in oxygen - there's not enough in there for a baby to breathe'). If the mother is getting adequate Vit D then there should be a minimum amount in breastmilk, but the vast majority of mothers don't. We're supposed to manufacture Vit D through exposure to sunlight, and modern lifestyles (most of our time indoors and use of sunscreen) means that people don't get enough of it anymore, which has quite negative health effects as Vit D is essential for a healthy immune system and strong bones, among other things.
So here is what I do:
I supplement my whole family. For babies under 6 months old (or not eating solid foods) I take higher supplements myself (4,000-6,000IU/day) which transmits enough through breastmilk to be the equivalent of 400IU/day (Study here). Once they're on solid foods, I give Carlson's Baby D supplement because it doesn not have any additional filler ingredients or vitamins. DS gets Solaray Sunny-D gummies or Animal Parade Vit D3 chewables - in winter he gets 1000iu/day and in summer (when he's outside more often) 500iu/day. DH and I both take 2000IU/day using Carlson Solar D Gems.
Taking D3 is important - that is the most easily bioavailable form of it.
I also don't always use sunscreen because I think humans are meant to be exposed to some amount of sunlight on a regular basis, there's a reason it feels so good to be in the sun. We use hats, we avoid peak sun strength hours, we go in and out of the shade, and if we're going to be swimming or otherwise exposed to sun for a long period of time in less clothing than normal, we use sunscreen.
I like a lot of what this article says on the topic of sun protection (what she's referring to with the 'sunscreen not working' is that up until the past decade, sunscreen did not block the most harmful rays - most of it does now with mixed success rates, and finding 'safe' sunscreen without harmful chemicals is very expensive).
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)